1. Field of the Invention
This invention relates to a substrate, and more particularly to a substrate for use in forming a ball grid array (BGA) package.
2. Description of the Related Art
As electronic devices have become more smaller and thinner, the velocity and the complexity of IC chip become more and more higher. Accordingly, a need has arisen for higher package efficiency. To meet the need, the ball grid array (BGA) technology has been developed by the semiconductor industry.
FIG. 1 is a cross sectional view of a typical BGA package 100 according to a preferred embodiment disclosed in U.S. Pat. No. 5,894,410. A semiconductor chip 101 is attached to a die covering area on an upper surface (die attach surface) 102a of a substrate 102 with adhesive. Bonding pads (not shown) on the chip 101 are connected with electrically conductive bond wires 103 to a ground ring 102b, a power ring 102c, and conductive traces 102d formed on the upper surface 102a of the substrate 102. Electrically conductive vias (not shown) are formed through the substrate 102 from the ground ring 102b, power ring 102c, or traces 102d on the upper surface 102a to a lower surface (mounting surface) 102e of the substrate 102 opposite the upper surface 102a. The lower surface 102e of the substrate 102 is provided with a plurality of solder pads 102f electrically connected to the ground ring 102b, power ring 102c, or corresponding traces 102d, respectively. Each solder pad 102f is provided with a solder ball 110 for making external electrical connection. The solder balls 110 include power balls for supplying the source voltage, ground balls for supplying the ground potential and signal balls. The ground ring 102b, power ring 102c, conductive traces 116 and solder pads 118 are usually made of metal with good electrical conductivity such as copper. The chip 101 and a portion of the upper surface 102a of the substrate 102 are encapsulated in a package body 120. Finally, the solder balls 110 are reflowed to attach the package 100 to a mounting board (not shown).
FIG. 2 is a bottom view of the package 100 of FIG. 1. The solder balls 110 of the package 100 are divided into an outer array and a center array. Typically, the semiconductor chip 101 is formed of microcrystalline silicon with a coefficient of thermal expansion (CTE) of 3-5 ppmxc2x0 C.xe2x88x921 and the substrate 102 is usually formed of polymer having a coefficient of thermal expansion of 20-30 ppmxc2x0 C.xe2x88x921. Since there is a significant difference between the semiconductor chip 101 and the substrate 102 in CTE, the semiconductor chip 101 and the substrate 102 expand and contract in different amounts along with temperature fluctuations. It has been found that, the differential thermal expansion between the semiconductor chip 101 and the substrate 102 will significantly increase the chances of failing solder joints in an area which corresponds to the periphery of the semiconductor chip 101. Therefore, it is desired to locate the solder balls 110 away from the periphery of the semiconductor chip 101 thereby reducing solder failure.
However, during manufacturing processes with rapid temperature increases such as IR reflow, the solder joints close to the periphery of the semiconductor chip 101 are still prone to fail.
The present invention therefore seeks to provide a BGA package which overcomes, or at least reduces the above-mentioned problems of the prior art.
It is a primary object of the present invention to provide a BGA package having a substrate with a specific contact pad distribution to enhance solder joint reliability.
The BGA package of the present invention mainly comprises a substrate and a semiconductor chip mounted thereon. The upper surface of the substrate comprises a ground ring. The substrate is characterized in that the lower surface thereof comprises an outer array of contact pads, a center array of contact pads and a plurality of intermediate pads located between the outer array of contact pads and the center array of contact pads, wherein all of the intermediate pads are electrically connected to the ground ring. A plurality of solder balls are attached to the pads of the substrate. The solder balls are typically reflowed to attach the package to a mounting board such as a printed circuit board. The semiconductor chip is electrically coupled to the solder balls by internal routing within the package.
Since the intermediate pads are designed for supplying ground potential, solder failure of some balls attached thereon is not critical. Therefore, the solder balls attached to the intermediate pads helps to absorb stress without sacrificing the reliability of the BGA package.